Conventionally, the surface illuminator using a cold cathode fluorescent tube as a light source is widely used as a backlight illumination in the liquid crystal display using a liquid crystal display panel. The cold cathode fluorescent tube used therein utilizes light emission of a phosphor (fluorescent material), and has a restriction in a displayable color reproduction range. Therefore, various examinations have been carried out for reproducing a further clear and natural tone. Above all, the surface illuminator for backlight using a light emitting diode (an LED element) of three colors such as red light (R-light), green light (G-light), and blue light (B-light) has been the focus. The light source using the three colors of R-light, G-light, and B-light or a multi-color LED element has a wider color reproduction range than that of the cold cathode fluorescent tube, thus making it possible to obtain a high image quality. Alternately, it is also possible to display an image by a field sequential method. Therefore, such developments are actively promoted.
For example, there is shown a structure in which, as the light source for display of the liquid crystal display using the LED elements of three colors of R-light, G-light, and B-light, the light source is divided into groups, and there is provided means for controlling to collectively turn on and turn off the light source for display for each group (see JP 2004-333583 A, for example). With this structure, a high luminance display and low power consumption are realized in the liquid crystal display not only of the field sequential method but also of a color filter method.
There is also shown a structure in which the LED elements of three colors of R-light, G-light, and B-light are arranged at three portions of an edge of an optical guide, so that each light be guided from each portion and be allowed to emit to an area corresponding to a pixel of each color of the liquid crystal display (see JP 2003-035904 A, for example). A color filter may be eliminated in this structure.
In addition, by using a semiconductor laser element having a luminance higher than that of the LED element and suitable for higher output, as a light emitting element of at least any one of the colors of three color light emitting elements, heat generation due to an increase of a drive current is suppressed and a fluctuation of characteristics is reduced. Note that this example specifically uses a red color semiconductor laser (see JP 2005-064163 A, for example).
As is shown in the example of JP 2005-064163 A, when a laser is used as the light source, it is important to realize a uniform luminance over a large area. When there is a variation in luminance, a displayed image is extremely deteriorated even though the color reproduction range becomes wider. However, there is almost no example of using a laser light source in backlight illumination, and JP 2005-064163 A neither describes nor discloses a way to realize a uniform luminance.
Further, in order to obtain an excellent image, suppression of a speckle noise is also required. The laser beams outputted from a laser has a significantly high coherence, and therefore mutual interference allows a random intensity variation to occur, thus forming the speckle noise. When the speckle noise occurs, deterioration of the image quality of the displayed image is invited. However, JP 2004-333583 A, JP 2003-035904 A, and JP 2005-064163 A mentioned above do not show a method of preventing the speckle noise, when the laser is used in the light source as the illuminator for backlight.
Note that, regarding a laser display and a laser exposure device, various studies have been carried out on the method of preventing the speckle noise. For example, in the method of preventing an occurrence of the speckle noise and making an intensity distribution of the laser beams uniform, by using an integrator lens, the integrator lens is reciprocally moved in a prescribed range (see JP 01-179908 A, for example).
JP 2004-333583 A and JP 2003-035904 A provide structures using a light emitting diode, and neither disclose nor suggest a structure and a method of performing uniform illumination over a large area by using a laser as a light source.
Further, JP 2005-064163 A describes an example of using a red color semiconductor laser, as a specific example. However in this example also, a specific method for obtaining a uniform planar illumination light by using the laser is not described at all.
Also, in the example of JP 01-179908 A, the occurrence of the speckle noise is prevented by reciprocally moving the integrator lens. However, it is impossible to use this structure disclosed therein as the illuminator for backlight of the liquid crystal display. Moreover, there is neither suggestion nor disclosure, regarding a point that the laser light source of three colors of R-light, G-light, and B-light is used.
Further, in a conventional large screen liquid crystal display, a plurality of light sources is required to be used in parallel for obtaining a sufficient luminance, because the luminance of the individual light source element is small. When a plurality of light sources is used, there is a problem that luminance unevenness is generated on its display surface, because individual initial luminance unevenness and the variation of characteristics by age-based change are individually different from each other.
In order to solve the above-described problems, the present invention is provided, and an object of the present invention is to provide a surface illuminator having a uniform luminance over a large area by using a laser light source, having a wide color reproduction range, and capable of suppressing a speckle noise, and a liquid crystal display using the same.